This is how you can format a disk on OpenBSD:
sysctl hw.disknames or dmesg | grep sd to find the names of attached disks.disklabel -E sdX where X is the name of the connected disk you want to edit (MAKE SURE YOU HAVE IT RIGHT!)z to delete the existing partition table. Then type a to add a partition. Afterwards w and then q to save and quit. newfs sdXa to format it.